In recent years, as an image formation method, an inkjet recording method has become popular because the inkjet recording method provides a simpler process and is easier to make a full-color image than other recording methods, and can provide a high-resolution image even with a simple device.
According to the inkjet recording method, a small amount of ink is jetted by foam generated by heat, or by pressure that is generated with the use of a piezoelectric or electrostatic force. The jetted ink is allowed to be adhered to a recording medium such as paper and then promptly dried or penetrated into the recording medium. In this manner, an image is formed. An application of the inkjet recording method has become widespread in various fields such as personal and industrial printers or printing.
For an inkjet recording device, an aqueous ink containing a water-soluble dye as a color material (a dye ink) is mainly used. However, the dye ink is disadvantageously poor in weather resistance and water resistance. Therefore, in recent years, a pigment ink containing a pigment instead of the water-soluble dye has been researched.
However, the pigment ink is poorer in color-developability, ink ejection stability, and storage stability than the dye ink. As the high-quality image technology for OA printers has advanced, higher image density is increasingly required even when recording is performed on plain paper with the pigment ink. However, in the case where the plain paper is used, there has been a problem that the pigment ink penetrates into the paper, resulting in a decrease in the pigment concentration on the paper surface and a drop in image density.
In recent years, in particular, a demand for industrial applications has been growing, and high-speed printing has been desired. For the high-speed printing, an inkjet printer equipped with a line head has been proposed. In the case of the high-speed printing, in order to accelerate the drying speed of the ink which has adhered to the recording medium, a penetrating agent is added to the ink to thereby allow water to penetrate into the recording medium. However, in this case, in addition to water, the pigment also penetrates into the recording medium in a high degree. As a result, the image density further decreases, which is problematic.
In the case of the plain paper, immediately after printing, water serving as an ink solvent makes a surface of the plain paper to swell, so that a difference between extension rates on a front surface and a back surface is increased to cause curling. This curling phenomenon has not been problematic in low-speed printing because it is eliminated as water is gradually dried. However, in the high-speed printing, the recording medium must be conveyed after printing in a state in which curling still remains thereon, which causes paper jam. An increase of an amount of a water-soluble organic solvent contained in the ink is effective for the curling, but, as a result, the ink is allowed to be more hydrophobic, making it difficult to ensure storage stability of the ink.
PTL 1 describes an ink containing an aqueous solvent, a pigment, a water-insoluble polymer dispersing agent, and a self-dispersing polymer which is excellent in ink ejection property and blocking resistance of an image. In addition, as a monomer of the water-insoluble polymer, a phosphate group-containing monomer is described. However, neither a copolymer containing a monomer represented by General Formula (5) of the present invention, nor its effects of improving dispersibility of the self-dispersing polymer and image density has been described.
PTL 2 describes a UV-curable white ink composition for inkjet recording which contains a while pigment, and a vinyl polymer containing a terminal substituent of an acid or salt thereof and a sulfonyl group or a phosphonyl group in molecule thereof. A terminal ammonium phosphate of a polymethacrylic acid derivative is exemplified as the polymer dispersing agent. However, neither a copolymer containing a monomer represented by the General Formula (5) of the present invention, nor its effect of improving image density in the case of using the polymer dispersing agent has been described.
PTL 3 describes improvement of fixability and image density by use of a combination of printing paper containing a polyvalent water-soluble metal salt, with an ink containing a pigment and a certain phosphate group having no surface activating ability. However, an effect of improving dispersibility of the ink and image density has not been described.